Large moving or rotating machinery requires preventive maintenance in order to schedule the replacement or repair of a part before costly breakdown and shutdown. The decision for scheduling and diagnosing the maintenance or repair is frequently made based on the new frequency or increased vibration level of the machinery. Existing vibration sensors cannot be easily placed on moving parts or in an inaccessible or small place. Technicians who scan the machinery take long time since their handheld sensor has to be attached to the machine for each point measurement. It is also desirable to scan a large part of machinery at once to speed up the diagnosis.
Radar vibration sensors have been proposed for such stand-off preventive maintenance assessment. However the calibration of such a sensor is difficult for several reasons. The magnitude of the sensor response depends on the distance to the target. The magnitude of the response is roughly proportional to distance. The magnitude of the sensor response may also vary by e.g. 6 dB because of signal cancellation due to small motion over short distance, e.g., one-half wavelength of carrier frequency e.g. 6.25 mm at 24 GHz. Still further, the magnitude of the sensor response depends on the reflectivity, i.e., radar cross section, of the target.